Wear behavior of coated surfaces has been studied extensively over the past 20 years, Although the behavior can be discussed in terms of classical models, it is described more completely using third-body concepts. These concepts consider the processes that take place at a moving inter-face : deformation and particle detachment, third-body formation and kinematics, and particle ejection. Tribological and analytical studies of several practical surface treatments, including TiN hard coatings and Mo-2 lubricant coatings, will be presented to illustrate the influence of third bodies on friction and wear behavior. More fundamental aspects of solid lubrication have also been studied in an ultrahigh vacuum chamber using gases to lubricate well-characterized surfaces. These studies indicate how third bodies form, how they influence friction and wear, and why some surface treatments are better than others. They also point to the need for better understanding of (1) the physical protection afforded by surface films, (2) tribochemical processes, and (3) chemical and mechanical properties of third bodies.